A speed reducer is a power transmission device that reduces the number of spins of a motor to a desired number of spins to obtain a higher torque by using gears. In conventional power and motion transmission devices, the speed reducer is widely used in various fields, such as ships, automobiles, heavy machinery used in construction, processing machines and automated manufacturing apparatus used in machinery industry, common home appliances, watches. The speed reducer can be applied from the large power transmission to the transmission of small load and precise angle.
Speed reducers currently used in robots are mainly gear speed reducers, harmonic speed reducers (harmonic drive), RV speed reducers and cycloid speed reducers. A harmonic drive has the advantages of a large transmission ratio, smooth operation, simple structure, and high transmission efficiency, and is an ideal gearing. However, the harmonic drive transmits load and motion through the elastic deformation of its flexible disc, so it is difficult to raise the rigidity, and the motion accuracy will be significantly reduced with time. The price of the harmonic drive is also too high.
As for the structures of the robots, a lot of rod movements are necessary, so the rod sizes and weights will affect its inertia and load capacity. If a compact speed reducer is employed, inertia can be reduced. In view of a lack of rigidity in harmonic drive and the size of apparent structure as well as the weight of the RV speed reducers, a small speed reducer with high reduction ratio is in earnest needs.